Referring now to Japanese Patent No. 57-55001, it will be seen that a prior art type of wheel-slip detection system is shown and disclosed therein. As shown in the patent, the velocity of each wheel axle is determined and the highest wheel velocity is designated as the standard velocity for comparison with the speed of each individual wheel axle to obtain the velocity difference between the wheel axles.
When the differential value of the velocity described above is obtained, the deceleration is determined by differentiating the velocity of each axle; and when each of these velocity differences or each differential value or each deceleration exceeds the set point, it is assumed that wheel slip is occurring. This slipping information is transmitted to the braking system and is used to control the force with which the brakes are applied. Each railroad car wheel also experiences the phenomenon of self-readhesion, which may occur without a reduction of the braking force, so that the slipping disappears naturally, even if the wheel-slip did start as a result of the application of the braking force. This phenomenon should also be taken advantage of as much as possible to shorten the distance required for braking. In the prior art system described above, the set point was relatively high, so as to broaden the area in which no response was made, with the result that slight degrees of slip could not be detected. However, when all the wheel axles experienced a slight degree of slip at the same time, the axle which is used to determine the standard velocity is also slipping. Thus, the velocity difference of each wheel axle from the standard velocity or the differential of these velocity differences, namely, the deceleration which differentiates the velocity of each axle, is relatively small. Therefore, a slight slip caused at the same time (called "sluggish slip" hereinafter) cannot be detected for a significant length of time by the existing system, since the set point is high. In the worst case, all the axles start to slip at once, and the braking distance increases. This can cause the outer thread of the wheel to be worn or flattened, which is a decided disadvantage.